1. Field of the Invention
The present invention relates to decentralized communication systems in which the beginning time and length of transmission is unknown, and in which there may be a number of nodes (users). The present invention is useful for coded, slow frequency hopped communication systems that operate in the presence of a deep fade or interference on the first channel used for transmission.
2. Description of Related Art
Communication systems are designed for particular environments. One environment is a plurality of stand-alone stations that communicate directly with each other without using any master controller whose transmissions can be received by all other stations in the system. In other words, this environment is a decentralized system including two or more stations that desire to communicate with each other. Any station may be capable of communicating with only a subset of the other stations in the system, and therefore the system is completely decentralized. An example of such an environment is an indoor communication system between stand-alone computer stations using radio frequency transmissions.
To communicate, simple conventional radio systems use a single dedicated radio frequency. However, the number of available radio frequencies is limited, which is particularly a problem for multi-user systems because only one station can transmit during any given time period. The likelihood of collisions between stations that use a single frequency is also very large. Furthermore, if the single radio frequency were to experience substantial fading or interference, a not uncommon occurrence, communication would be impossible.
For multi-user systems, it has been suggested to use a number (N) of radio frequency channels that are transmitted in a particular sequence known to the receiving stations. The technique of sequencing through N channels, when combined with coding techniques to be discussed, is called "cyclical slow frequency hopping and coding", and can be a powerful technique to combat fading and interference in a wireless network. This technique is described in detail in "Indoor Radio Communications Using Time-Division Multiple Access with Cyclical Slow Frequency Hopping and Coding", by Adel A. M. Saleh and Leonard J. Cimini, Jr., IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, Jan. 1989, pp. 59-70.
In a system using cyclical slow frequency hopping and coding, data to be transmitted is divided into segments, and each segment is transmitted on a different carrier frequency (channel). Burst error correction is used both within and across segments when the data is re-assembled, thereby permitting the data to be correctly reconstituted even in the presence of fading or interference on one or more channels, even if up to e segments out of the N are corrupted.
One problem with decentralized systems is that the receiving stations have no foreknowledge that a transmission will begin. Therefore the receiving stations do not know when to begin receiving a transmission. One important prerequisite to effective use of slow frequency hopping with coding is that the presence of a transmission must be detected with high probability. If the transmission's presence is difficult to detect, then many messages would be missed and the theoretical gains from slow frequency hopping with coding would not be realized.
Conventionally, receiving stations monitor the first channel, and when a transmission is received, they begin receiving the data and continue cycling through each of the known frequencies until the transmission is complete. One problem with monitoring the first channel is detecting the presence of the desired transmission in the presence of a deep fade or interference on the first or subsequent channels. If there were a deep fade or interference such that a transmission on the first channel could not be detected, the entire transmission would be lost even though the first channel might have been the only impaired channel. In this case, the error correction techniques that enable the transmission to be recovered even when e channels are impaired would be of no value, and the system robustness would be dramatically decreased from its theoretical level.
Thus, it would be advantageous if the presence of a desired transmission could be detected and all recoverable segments received despite the presence of a deep fade or strong interference on the first channel in the sequence.
The indoor radio environment is characterized by potentially severe multipath fading, together with large propagation losses. Even with a dedicated radio frequency band, there are potentially serious problems with interference from other devices that emit radio frequency energy on the same channel during operation. Multipath fading causes the signal-to-noise ratio (SNR) vs. bit-error ratio (BER) curves to differ significantly from their free-space characteristics. It has been experimentally determined that the indoor radio environment at about 1.0 GHz can be modeled as a very slowly varying, frequency-selective, Rayleigh fading environment. The movement of people, objects, etc. within the building causes the signal amplitude at a point to be slowly varying.
One conventional medium access control (MAC) protocol for a decentralized Local Area Network (LAN), in which all nodes are peers, is a variant of Carrier Sense Multiple Access (CSMA). The channel access protocol of CSMA is simple and has low overhead. Token passing protocols, in which one or more nodes are dynamically designated as a master node on the LAN, are not suitable for this environment, given the possibility of hidden nodes and the (semi) mobile nature of the nodes on the LAN.
To avoid collisions in networks, a station employing CSMA will not transmit if it knows that another station is transmitting. This station will wait until the other's transmission is complete before beginning transmission. This system is useful for networks in which the stations are able to detect that the others are transmitting. However, this system will not avoid collisions in the presence of "hidden nodes". The "hidden node" problem results when a node is visible to one node, but not to another. A hidden node is always a problem even with friendly systems (stations using like protocols) because the hidden nodes can interfere with transmissions by other nodes, in spite of the nodes' attempts to avoid mutual interference. Suppose that there is a multi-station decentralized communication system having stations including station A and station B. These two stations cannot hear each other, and both wish to transmit. Even if station A starts transmitting well before station B, station B will not hear the transmission, and will start to transmit. All stations in the intersection of the transmission radius of the two transmitting stations will hear both signals and there will be a collision if either packet was addressed to a station within this intersection.
Unfortunately, in the presence of hidden nodes, CSMA performance degrades rapidly and significantly to ALOHA performance. Collision detect systems, such as that used in Ethernet, are not feasible in the radio environment, and would not help with hidden nodes. Collision Avoidance using state-following, as in Localtalk, is not a practical option. The Busy Tone Multiple Access method of achieving collision avoidance has large associated costs because it requires a form of full-duplex operation.
For any practical decentralized communication system, the problem of hidden nodes must be addressed. Furthermore, the effect of external interference must also be minimized.